The use of an epitaxial base in a BJT can provide the advantage of increased control over base doping concentrations, which results in narrower bases with reduced transit times and lower peripheral base dopant concentrations for reduced base-emitter capacitances (Cje). These characteristics generally enhance the performance of the resulting device. However, epitaxial bases are difficult to integrate into double polysilicon self-aligned (DPSA) BJT's. If the epitaxial base is deposited prior to the base polysilicon, then the epitaxial base can be damaged or removed by the base polysilicon etch. Alternatively, deposition of the epitaxial base after the emitter region etch requires selective epitaxy processing. Even with selective epitaxy processing, side-wall depositions on the base polysilicon can affect the final emitter region sizing.
One remedy for this issue is to deposit the epitaxial layer and pattern a silicon dioxide (SiO.sub.2) etch stop prior to the base polysilicon deposition. The emitter opening can then be formed without damaging the underlying epitaxial layer. The structure resulting from this double polysilicon non self-aligned (DPNSA) process is illustrated in FIG. 1.
In this known structure, the base diffusion (originating from the polysilicon-epitaxy interface) is needed to reduce the higher resistance base epitaxy region in order to achieve a low resistance base layer for desired contact performance. This base diffusion cannot not overlap the emitter diffusion (originating from the emitter polysilicon-base epitaxy interface) without significantly increasing Cje, reducing base-emitter breakdown (BVebo) and negatively impacting reliability. Since the extrinsic base diffusion process is separated from the base contact by the size of the overlap of the etch-stop region over the emitter region, and since this overlap is affected by lithographic sizing variations as well as alignment errors, the drawn overlap must be equal to the total of the widths due to the base diffusion, emitter diffusion, sizing variation, and maximum alignment error. For this reason, the base link-up resistance and the total device area will be larger than achieved in a standard DPSA structure.
A DPNSA BJT is shown in FIG. 1, and a dual poly self-aligned (DPSA) BJT is shown in FIG. 2. In the DPNSA BJT of FIG. 1, the distance between the inside edge 20 (outside edge of the emitter window) of the base-emitter spacer 22 to the edge 24 of the isolation structure 26 is designed to include the base link-up diffusion width (or spacer width) "A," the emitter to etch stop alignment "B," the pad to field alignment "C," and the minimum width for extrinsic base diffusion (or extrinsic base junction depth) "D." Each of these dimensions is approximately 0.1 microns, requiring the edge of the emitter window to be positioned at a minimum of 0.4 microns away from the border 24 of the isolation structure 26 to allow for maximum deviation in processing.
In the DPSA BJT shown in FIG. 2, the minimum spacing between the inside edge 28 of the spacer 30 (outside edge of the emitter window) is spaced from the edge 32 of isolation structure 34 by the base link-up diffusion width (spacer width) "A," the emitter to field alignment "E," and the minimum width for extrinsic base diffusion (or extrinsic base junction depth) "D." As each of these dimensions is approximately 0.1 microns, the placement of the edge of the emitter window is designed to allow 0.3 microns variance between its placement and the isolation structure to allow for deviation in the processing.
It is with the foregoing problems in mind that the instant invention was developed.